This is a fulacht fia, a type of ancient cooking site found in remarkable numbers across Ireland, typically dating from the Bronze Age. The characteristic form is a crescent or horseshoe of scorched and shattered stone, built up over time as heated rocks were used to boil water in a trough, then discarded when they cracked. The mound at Knockduff measures eight metres by seven metres, rises to about 0.6 metres at its highest point, and is lower on its southern side. Its opening, roughly four metres wide, faces north-northeast.

What gives this particular site a quiet extra dimension is that it does not stand alone. A second fulacht fia lies approximately two hundred metres to the west-northwest. The clustering of these sites is not unusual; fulachtaí fia tend to appear near water sources and in low-lying, damp ground, precisely the kind of terrain that would have made a reliable trough easy to maintain. Finding two within easy walking distance of each other hints at repeated or prolonged use of this part of the landscape, though whether that reflects seasonal activity, communal gathering, or simple convenience of the terrain is something the mounds themselves cannot say.